Step Voltage Method for Insulation Testing [PDF]

In the realm of electrical maintenance, uncovering hidden issues within insulation is crucial for the safety and reliability of equipment. One valuable technique for this purpose is the step-voltage method. In this method, you use a multi-voltage Megger instrument to apply two or more voltages in steps, such as 500 volts followed by 1000 volts. The goal is to observe any reduction in insulation resistance at the higher voltage, as this can indicate insulation weaknesses that only manifest at elevated voltages.

Step Voltage Method for Insulation Testing

Let’s delve deeper into the step-voltage method and its significance:

The Method in Action: Imagine you are testing the insulation of a piece of equipment. First, you perform a test at a lower voltage, like 500 volts, and record the insulation resistance. After discharging the sample, you conduct another test, this time at a higher voltage, such as 2500 volts. Any difference in the two tests, in terms of megohms, signals potential weaknesses that warrant further investigation.

Understanding the Theory: The concept behind the step-voltage technique is somewhat intricate, but we’ll simplify it. Moisture and dirt in insulation are usually detectable at lower test voltages, far below what the equipment encounters in regular service. However, when you gradually increase the voltage, reaching or surpassing typical operational levels, local weak points in the insulation begin to exert more influence. These weak spots tend to exhibit a rapid decrease in resistance as the electrical stress on them intensifies. This drop in resistance becomes evident when comparing consecutive Megger instrument readings.

Plotting the Results: By plotting the results, you can clearly see a sharp drop in resistance as the electrical stress approaches or exceeds real-world conditions. A significant reduction in megohms and a less steep upward slope in the higher-voltage plot indicate potential issues. This visual representation helps you identify insulation problems effectively.

These two test curves exemplify the step-voltage method’s ability to differentiate between good and bad insulation conditions. While Curve 1 reveals a concerning drop in resistance as voltage increases, indicating an insulation problem, Curve 2 showcases the positive outcome of effective maintenance, where insulation resistance remains stable even at higher test voltages. These visual representations emphasize the method’s practical utility in assessing and improving insulation reliability.

Key Considerations: In the step-voltage method, you only need to maintain a constant test voltage between steps for about 60 seconds, a period that won’t affect the trend in resistance change. The timed period should always remain consistent for a given piece of equipment. Your focus is on the change in resistance, not the absolute resistance values, making the results independent of insulation material and temperature. Like spot-reading and time-resistance measurements, this method yields its full value when performed periodically or as part of a scheduled maintenance routine.

Moisture Detection: The step-voltage method excels at detecting excessive moisture or contaminants in the insulation of equipment rated at voltages equal to or greater than your multi-voltage Megger instrument’s highest voltage. For instance, suppose you initially test insulation resistance at 500 VDC and then at 2500 VDC. Even though the latter voltage may not stress the insulation beyond its rating, a lower insulation resistance at the higher test voltage often indicates the presence of contaminants or moisture.

The Voltage Ratio: For optimal results, it’s advisable to apply the voltages in a ratio of 1 to 5 or greater (e.g., 500 and 2500 volts). Experience shows that a 25% change in insulation resistance value, with a 1-to-5 ratio in test voltages, often points to the presence of excessive moisture or contaminants.

In summary, the step-voltage method is a potent tool in your electrical maintenance toolkit. By following these principles, you can uncover insulation weaknesses that may not be apparent at lower voltages, ensuring the reliability and safety of your electrical systems. Whether you’re a seasoned professional or just starting, this technique empowers you to elevate your maintenance practices.

Related Articles:

What is Good insulation?
What Makes Insulation Go Bad?
How Insulation Resistance is Measured.
How to Interpret Resistance Readings.
Factors Affecting Insulation Resistance Readings.
Types of Insulation Resistance Tests.
Test Voltage vs. Equipment Rating.
AC Testing vs. DC.
Use of DC Dielectric Test Set.
Tests During Drying out of Equipment.
Effect of Temperature on insulation Resistance.
Effects of humidity.
Preparation of Apparatus to test.
Safety Precautions.
Connections for testing insulation resistance of electrical equipment.
Additional Notes About using A Megger Insulation Tester.
Interpretation-Minimum Values.
Minimum Values for Insulation Resistance.
Tests Using Multi-Voltage Megger Insulation Testers.
Use of a Guard Terminal.
Outdoor Oil Circuit Breaker.